PREPARATION OF CULTURE MEDIA I5 



the fire has been raked out at night. In the morning the mass 

 will be found to have solidified, and there will be a coagulum at 

 the bottom. The beaker is then inverted and the mass " turned 

 out " just as a cook turns out a jelly, and the sediment is cut 

 off with a sharp knife. This avoids filtration, which is very 

 troublesome. 



An alternative method is to filter the melted jelly through 

 moistened filter-paper. It is necessary to keep flask and funnel in 

 a steamer (the water of which is kept boiling vigorously) during 

 the whole process, or the jelly will solidify in the outflow tube of 

 the funnel. Or it may be filtered through a double thickness of 

 ordinary surgical lint (non-medicated). It runs through this very 

 quickly, and the funnel need not be kept hot. The resulting 

 medium is not absolutely clear, but sufficiently so for most 

 purposes. 



The agar is again melted and placed in test-tubes ; these are 

 sterilized on three successive days and allowed to set in a sloping 

 position. For certain purposes glucose, glycerin, etc., are added 

 to the agar. The addition should be made to the melted medium 

 just before it is poured into the tubes. 



Solidified Blood-serum is very difficult to prepare, and is 

 best purchased ready for use from a good laboratory. It is used 

 chiefly in the diagnosis of diphtheria by the examination of 

 " swabs " from the throat. 



Potato tubes are in occasional use, and are easy to prepare. 

 The process is as follows : Take large and sound potatoes and 

 scrub them thoroughly with a nail-brush under the tap. Peel 

 them deeply enough to remove the eyes completely. Then cut 

 them into cylinders a little less than | inch in diameter (if you are 

 using |-inch test-tubes) and as long as possible ; this is best done 

 by means of a cork-borer, but they may be shaped by means of a 

 knife if this is not at hand. Then cut each cylinder in half by a 

 cut running obliquely from end to end ; the shape of each half 

 should be exactly like that of the medium in a sloped gelatin tube. 

 Place the halves in a large vessel of tap-water and allow them 

 to soak all night ; it is a good plan to use running water if 

 possible. 



After this has been done place each half (base downwards) in a 

 test-tube, having previously inserted a small mass of absorbent 

 cotton-wool and enough water to saturate it. Plug the mouth of 

 the tube with cotton-wool and sterilize on three successive days. 



